The purpose of the proposed project is to examine the potential physiological role(s) for gamma-aminobutyric acid and substance P in regulation of excitability of physiologically and morphologically identified primary afferent fibers and interneurons on the superficial parts of the mammalian spinal dorsal horn (laminae I-III of Rexed). Conventional approach will be to use cat spinal cord in situ and apply GABA and substance P iontophoretically or by micropressure injections into the laminae I-III of the dorsal horn, while testing the electrical excitability of single sural A- and C-afferent fibers or performing intracellular recording from the large diameter primary afferents or functionally identified interneurons or dorsal root ganglion cells. Ionic mechanisms of presynaptic and postsynaptic actions of GABA and substance P will be investigated in vitro by using the rat spinal cord slice preparation. Morphological identification of physiologically and pharmacologically characterized dorsal horn interneurons and primary afferent neurons will be done by intracellular labeling with horseradish peroxidase.